Measurement of indirect CP-violating asymmetries in
D[superscript 0] K[superscript +]K[superscript ] and
D[superscript 0] [superscript +][superscript ] decays at
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Citation
Aaltonen, T., et al. "Measurement of indirect CP-violating
asymmetries in D[superscript 0] K[superscript +]K[superscript ]
and D[superscript 0] [superscript +][superscript ] decays at
CDF." Phys. Rev. D 90, 111103 (December 2014). © 2014
American Physical Society
As Published
http://dx.doi.org/10.1103/PhysRevD.90.111103
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American Physical Society
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Final published version
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Wed May 25 15:50:34 EDT 2016
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http://hdl.handle.net/1721.1/92812
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RAPID COMMUNICATIONS
PHYSICAL REVIEW D 90, 111103(R) (2014)
Measurement of indirect CP-violating asymmetries in D0 → K þ K −
and D0 → πþ π− decays at CDF
T. Aaltonen,21 S. Amerio,39a,39b D. Amidei,31 A. Anastassov,15,v A. Annovi,17 J. Antos,12 G. Apollinari,15 J. A. Appel,15
T. Arisawa,52 A. Artikov,13 J. Asaadi,47 W. Ashmanskas,15 B. Auerbach,2 A. Aurisano,47 F. Azfar,38 W. Badgett,15 T. Bae,25
A. Barbaro-Galtieri,26 V. E. Barnes,43 B. A. Barnett,23 P. Barria,41a,41c P. Bartos,12 M. Bauce,39a,39b F. Bedeschi,41a
S. Behari,15 G. Bellettini,41a,41b J. Bellinger,54 D. Benjamin,14 A. Beretvas,15 A. Bhatti,45 K. R. Bland,5 B. Blumenfeld,23
A. Bocci,14 A. Bodek,44 D. Bortoletto,43 J. Boudreau,42 A. Boveia,11 L. Brigliadori,6a,6b C. Bromberg,32 E. Brucken,21
J. Budagov,13 H. S. Budd,44 K. Burkett,15 G. Busetto,39a,39b P. Bussey,19 P. Butti,41a,41b A. Buzatu,19 A. Calamba,10
S. Camarda,4 M. Campanelli,28 F. Canelli,11,cc B. Carls,22 D. Carlsmith,54 R. Carosi,41a S. Carrillo,16,l B. Casal,9,j
M. Casarsa,48a A. Castro,6a,6b P. Catastini,20 D. Cauz,48a,48b,48c V. Cavaliere,22 A. Cerri,26,e L. Cerrito,28,q Y. C. Chen,1
M. Chertok,7 G. Chiarelli,41a G. Chlachidze,15 K. Cho,25 D. Chokheli,13 A. Clark,18 C. Clarke,53 M. E. Convery,15
J. Conway,7 M. Corbo,15,y M. Cordelli,17 C. A. Cox,7 D. J. Cox,7 M. Cremonesi,41a D. Cruz,47 J. Cuevas,9,x R. Culbertson,15
N. d’Ascenzo,15,u M. Datta,15,ff P. de Barbaro,44 L. Demortier,45 M. Deninno,6a M. D’Errico,39a,39b F. Devoto,21
A. Di Canto,41a,41b B. Di Ruzza,15,p J. R. Dittmann,5 S. Donati,41a,41b M. D’Onofrio,27 M. Dorigo,48a,48d A. Driutti,48a,48b,48c
K. Ebina,52 R. Edgar,31 A. Elagin,47 R. Erbacher,7 S. Errede,22 B. Esham,22 S. Farrington,38 J. P. Fernández Ramos,29
R. Field,16 G. Flanagan,15,s R. Forrest,7 M. Franklin,20 J. C. Freeman,15 H. Frisch,11 Y. Funakoshi,52 C. Galloni,41a,41b
A. F. Garfinkel,43 P. Garosi,41a,41c H. Gerberich,22 E. Gerchtein,15 S. Giagu,46a V. Giakoumopoulou,3 K. Gibson,42
C. M. Ginsburg,15 N. Giokaris,3 P. Giromini,17 V. Glagolev,13 D. Glenzinski,15 M. Gold,34 D. Goldin,47 A. Golossanov,15
G. Gomez,9 G. Gomez-Ceballos,30 M. Goncharov,30 O. González López,29 I. Gorelov,34 A. T. Goshaw,14 K. Goulianos,45
E. Gramellini,6a C. Grosso-Pilcher,11 R. C. Group,51,15 J. Guimaraes da Costa,20 S. R. Hahn,15 J. Y. Han,44 F. Happacher,17
K. Hara,49 M. Hare,50 R. F. Harr,53 T. Harrington-Taber,15,m K. Hatakeyama,5 C. Hays,38 J. Heinrich,40 M. Herndon,54
A. Hocker,15 Z. Hong,47 W. Hopkins,15,f S. Hou,1 R. E. Hughes,35 U. Husemann,55 M. Hussein,32,aa J. Huston,32
G. Introzzi,41a,41e,41f M. Iori,46a,46b A. Ivanov,7,o E. James,15 D. Jang,10 B. Jayatilaka,15 E. J. Jeon,25 S. Jindariani,15
M. Jones,43 K. K. Joo,25 S. Y. Jun,10 T. R. Junk,15 M. Kambeitz,24 T. Kamon,25,47 P. E. Karchin,53 A. Kasmi,5 Y. Kato,37,n
W. Ketchum,11,gg J. Keung,40 B. Kilminster,15,cc D. H. Kim,25 H. S. Kim,25 J. E. Kim,25 M. J. Kim,17 S. H. Kim,49
S. B. Kim,25 Y. J. Kim,25 Y. K. Kim,11 N. Kimura,52 M. Kirby,15 K. Knoepfel,15 K. Kondo,52,* D. J. Kong,25 J. Konigsberg,16
A. V. Kotwal,14 M. Kreps,24 J. Kroll,40 M. Kruse,14 T. Kuhr,24 M. Kurata,49 A. T. Laasanen,43 S. Lammel,15 M. Lancaster,28
K. Lannon,35,w G. Latino,41a,41c H. S. Lee,25 J. S. Lee,25 S. Leo,22 S. Leone,41a J. D. Lewis,15 A. Limosani,14,r E. Lipeles,40
A. Lister,18,a H. Liu,51 Q. Liu,43 T. Liu,15 S. Lockwitz,55 A. Loginov,55 D. Lucchesi,39a,39b A. Lucà,17 J. Lueck,24
P. Lujan,26 P. Lukens,15 G. Lungu,45 J. Lys,26 R. Lysak,12,d R. Madrak,15 P. Maestro,41a,41c S. Malik,45 G. Manca,27,b
A. Manousakis-Katsikakis,3 L. Marchese,6a,hh F. Margaroli,46a P. Marino,41a,41d K. Matera,22 M. E. Mattson,53
A. Mazzacane,15 P. Mazzanti,6a R. McNulty,27,i A. Mehta,27 P. Mehtala,21 C. Mesropian,45 T. Miao,15 D. Mietlicki,31
A. Mitra,1 H. Miyake,49 S. Moed,15 N. Moggi,6a C. S. Moon,15,y R. Moore,15,dd,ee M. J. Morello,41a,41d A. Mukherjee,15
Th. Muller,24 P. Murat,15 M. Mussini,6a,6b J. Nachtman,15,m Y. Nagai,49 J. Naganoma,52 I. Nakano,36 A. Napier,50 J. Nett,47
C. Neu,51 T. Nigmanov,42 L. Nodulman,2 S. Y. Noh,25 O. Norniella,22 L. Oakes,38 S. H. Oh,14 Y. D. Oh,25 I. Oksuzian,51
T. Okusawa,37 R. Orava,21 L. Ortolan,4 C. Pagliarone,48a E. Palencia,9,e P. Palni,34 V. Papadimitriou,15 W. Parker,54
G. Pauletta,48a,48b,48c M. Paulini,10 C. Paus,30 T. J. Phillips,14 E. Pianori,40 J. Pilot,7 K. Pitts,22 C. Plager,8 L. Pondrom,54
S. Poprocki,15,f K. Potamianos,26 A. Pranko,26 F. Prokoshin,13,z F. Ptohos,17,g G. Punzi,41a,41b I. Redondo Fernández,29
P. Renton,38 M. Rescigno,46a F. Rimondi,6a,* L. Ristori,41a,15 A. Robson,19 T. Rodriguez,40 S. Rolli,50,h M. Ronzani,41a,41b
R. Roser,15 J. L. Rosner,11 F. Ruffini,41a,41c A. Ruiz,9 J. Russ,10 V. Rusu,15 W. K. Sakumoto,44 Y. Sakurai,52 L. Santi,48a,48b,48c
K. Sato,49 V. Saveliev,15,u A. Savoy-Navarro,15,y P. Schlabach,15 E. E. Schmidt,15 T. Schwarz,31 L. Scodellaro,9 F. Scuri,41a
S. Seidel,34 Y. Seiya,37 A. Semenov,13 F. Sforza,41a,41b S. Z. Shalhout,7 T. Shears,27 P. F. Shepard,42 M. Shimojima,49,t
M. Shochet,11 I. Shreyber-Tecker,33 A. Simonenko,13 K. Sliwa,50 J. R. Smith,7 F. D. Snider,15 H. Song,42 V. Sorin,4
R. St. Denis,19,* M. Stancari,15 D. Stentz,15,v J. Strologas,34 Y. Sudo,49 A. Sukhanov,15 I. Suslov,13 K. Takemasa,49
Y. Takeuchi,49 J. Tang,11 M. Tecchio,31 P. K. Teng,1 J. Thom,15,f E. Thomson,40 V. Thukral,47 D. Toback,47
S. Tokar,12 K. Tollefson,32 T. Tomura,49 D. Tonelli,15,e S. Torre,17 D. Torretta,15 P. Totaro,39a M. Trovato,41a,41d
F. Ukegawa,49 S. Uozumi,25 G. Velev,15 C. Vellidis,15 C. Vernieri,41a,41d M. Vidal,43 R. Vilar,9 J. Vizán,9,bb
M. Vogel,34 G. Volpi,17 F. Vázquez,16,l P. Wagner,40 R. Wallny,15,j S. M. Wang,1 D. Waters,28 W. C. Wester III,15
D. Whiteson,40,c A. B. Wicklund,2 S. Wilbur,7 H. H. Williams,40 J. S. Wilson,31 P. Wilson,15 B. L. Winer,35
P. Wittich,15,f S. Wolbers,15 H. Wolfe,35 T. Wright,31 X. Wu,18 Z. Wu,5 K. Yamamoto,37 D. Yamato,37 T. Yang,15
U. K. Yang,25 Y. C. Yang,25 W.-M. Yao,26 G. P. Yeh,15 K. Yi,15,m J. Yoh,15 K. Yorita,52 T. Yoshida,37,k G. B. Yu,14 I. Yu,25
A. M. Zanetti,48a Y. Zeng,14 C. Zhou,14 and S. Zucchelli6a,6b
(CDF Collaboration)
1550-7998=2014=90(11)=111103(8)
111103-1
© 2014 American Physical Society
RAPID COMMUNICATIONS
T. AALTONEN et al.
PHYSICAL REVIEW D 90, 111103(R) (2014)
1
Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2
Argonne National Laboratory, Argonne, Illinois 60439, USA
3
University of Athens, 157 71 Athens, Greece
4
Institut de Fisica d’Altes Energies, ICREA, Universitat Autonoma de Barcelona,
E-08193 Bellaterra (Barcelona), Spain
5
Baylor University, Waco, Texas 76798, USA
6a
Istituto Nazionale di Fisica Nucleare Bologna, Italy
6b
University of Bologna, I-40127 Bologna, Italy
7
University of California, Davis, Davis, California 95616, USA
8
University of California, Los Angeles, Los Angeles, California 90024, USA
9
Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
10
Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
11
Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
12
Comenius University, 842 48 Bratislava, Slovakia and Institute of Experimental Physics,
040 01 Kosice, Slovakia
13
Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
14
Duke University, Durham, North Carolina 27708, USA
15
Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
16
University of Florida, Gainesville, Florida 32611, USA
17
Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
18
University of Geneva, CH-1211 Geneva 4, Switzerland
19
Glasgow University, Glasgow G12 8QQ, United Kingdom
20
Harvard University, Cambridge, Massachusetts 02138, USA
21
Division of High Energy Physics, Department of Physics, University of Helsinki, FIN-00014 Helsinki,
Finland and Helsinki Institute of Physics, FIN-00014 Helsinki, Finland
22
University of Illinois, Urbana, Illinois 61801, USA
23
The Johns Hopkins University, Baltimore, Maryland 21218, USA
24
Institut für Experimentelle Kernphysik, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany
25
Center for High Energy Physics: Kyungpook National University, Daegu 702-701, Korea;
Seoul National University, Seoul 151-742, Korea; Sungkyunkwan University, Suwon 440-746, Korea;
Korea Institute of Science and Technology Information, Daejeon 305-806, Korea; Chonnam National
University, Gwangju 500-757, Korea; Chonbuk National University, Jeonju 561-756, Korea; and Ewha
Womans University, Seoul 120-750, Korea
26
Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
27
University of Liverpool, Liverpool L69 7ZE, United Kingdom
28
University College London, London WC1E 6BT, United Kingdom
29
Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
30
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
31
University of Michigan, Ann Arbor, Michigan 48109, USA
32
Michigan State University, East Lansing, Michigan 48824, USA
33
Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
34
University of New Mexico, Albuquerque, New Mexico 87131, USA
35
The Ohio State University, Columbus, Ohio 43210, USA
36
Okayama University, Okayama 700-8530, Japan
37
Osaka City University, Osaka 558-8585, Japan
38
University of Oxford, Oxford OX1 3RH, United Kingdom
39a
Istituto Nazionale di Fisica Nucleare, Sezione di Padova, Italy
39a
University of Padova, I-35131 Padova, Italy
40
University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
41a
Istituto Nazionale di Fisica Nucleare Pisa, Italy
41b
University of Pisa, Italy
41c
University of Siena, Italy
41d
Scuola Normale Superiore, I-56127 Pisa, Italy
41e
INFN Pavia, I-27100 Pavia, Italy
41f
University of Pavia, I-27100 Pavia, Italy
42
University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
43
Purdue University, West Lafayette, Indiana 47907, USA
44
University of Rochester, Rochester, New York 14627, USA
45
The Rockefeller University, New York, New York 10065, USA
46a
Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, Italy
111103-2
RAPID COMMUNICATIONS
MEASUREMENT OF INDIRECT CP-VIOLATING …
PHYSICAL REVIEW D 90, 111103(R) (2014)
46b
Sapienza Università di Roma, I-00185 Roma, Italy
Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University,
College Station, Texas 77843, USA
48a
Istituto Nazionale di Fisica Nucleare Trieste, Italy
48b
Gruppo Collegato di Udine, Italy
48c
University of Udine, I-33100 Udine, Italy
48d
University of Trieste, I-34127 Trieste, Italy
49
University of Tsukuba, Tsukuba, Ibaraki 305, Japan
50
Tufts University, Medford, Massachusetts 02155, USA
51
University of Virginia, Charlottesville, Virginia 22906, USA
52
Waseda University, Tokyo 169, Japan
53
Wayne State University, Detroit, Michigan 48201, USA
54
University of Wisconsin, Madison, Wisconsin 53706, USA
55
Yale University, New Haven, Connecticut 06520, USA
(Received 20 October 2014; published 30 December 2014)
47
We report a measurement of the indirect CP-violating asymmetries (AΓ ) between effective lifetimes of
anticharm and charm mesons reconstructed in D0 → K þ K − and D0 → π þ π − decays. We use the full data
set of proton-antiproton collisions collected by the Collider Detector at Fermilab experiment and
corresponding to 9.7 fb−1 of integrated luminosity. The strong-interaction decay Dþ → D0 π þ is used
to identify the meson at production as D0 or D̄0. We statistically subtract D0 and D̄0 mesons originating
from b-hadron decays and measure the yield asymmetry between anticharm and charm decays as a function
of decay time. We measure AΓ ðK þ K − Þ ¼ ð−0.19 0.15ðstatÞ 0.04ðsystÞÞ% and AΓ ðπ þ π − Þ ¼
ð−0.01 0.18ðstatÞ 0.03ðsystÞÞ%. The results are consistent with the hypothesis of CP symmetry
and their combination yields AΓ ¼ ð−0.12 0.12Þ%.
DOI: 10.1103/PhysRevD.90.111103
PACS numbers: 13.25.Ft, 11.30.Er, 14.40.Lb
*
Deceased.
With visitor from University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada.
With visitor from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy.
c
With visitor from University of California Irvine, Irvine, California 92697, USA.
d
With visitor from Institute of Physics, Academy of Sciences of the Czech Republic, 182 21, Czech Republic.
e
With visitor from CERN, CH-1211 Geneva, Switzerland.
f
With visitor from Cornell University, Ithaca, New York 14853, USA.
g
With visitor from University of Cyprus, Nicosia CY-1678, Cyprus.
h
With visitor from Office of Science, U.S. Department of Energy, Washington, DC 20585, USA.
i
With visitor from University College Dublin, Dublin 4, Ireland.
j
With visitor from ETH, 8092 Zürich, Switzerland.
k
With visitor from University of Fukui, Fukui City, Fukui Prefecture 910-0017, Japan.
l
With visitor from Universidad Iberoamericana, Lomas de Santa Fe, México C.P. 01219, Distrito Federal.
m
With visitor from University of Iowa, Iowa City, Iowa 52242, USA.
n
With visitor from Kinki University, Higashi-Osaka City 577-8502, Japan.
o
With visitor from Kansas State University, Manhattan, Kansas 66506, USA.
p
With visitor from Brookhaven National Laboratory, Upton, New York 11973, USA.
q
With visitor from Queen Mary, University of London, London E1 4NS, United Kingdom.
r
With visitor from University of Melbourne, Victoria 3010, Australia.
s
With visitor from Muons, Inc., Batavia, Illinois 60510, USA.
t
With visitor from Nagasaki Institute of Applied Science, Nagasaki 851-0193, Japan.
u
With visitor from National Research Nuclear University, Moscow 115409, Russia.
v
With visitor from Northwestern University, Evanston, Illinois 60208, USA.
w
With visitor from University of Notre Dame, Notre Dame, Indiana 46556, USA.
x
With visitor from Universidad de Oviedo, E-33007 Oviedo, Spain.
y
With visitor from CNRS-IN2P3, Paris F-75205, France.
z
With visitor from Universidad Tecnica Federico Santa Maria, 110v Valparaiso, Chile.
aa
With visitor from The University of Jordan, Amman 11942, Jordan.
bb
With visitor from Universite catholique de Louvain, 1348 Louvain-La-Neuve, Belgium.
cc
With visitor from University of Zürich, 8006 Zürich, Switzerland.
dd
With visitor from Massachusetts General Hospital, Boston, Massachusetts 02114, USA.
ee
With visitor from Harvard Medical School, Boston, Massachusetts 02114, USA.
ff
With visitor from Hampton University, Hampton, Virginia 23668, USA.
gg
With visitor from Los Alamos National Laboratory, Los Alamos, New Mexico 87544, USA.
hh
With visitor from Università degli Studi di Napoli Federico I, I-80138 Napoli, Italy.
a
b
111103-3
RAPID COMMUNICATIONS
T. AALTONEN et al.
PHYSICAL REVIEW D 90, 111103(R) (2014)
The noninvariance of the laws of physics under the
simultaneous transformations of parity and charge conjugation (CP violation) is described in the standard model
(SM) through an irreducible complex phase in the weakinteraction couplings of quarks. A broad class of SM
extensions allows for additional sources of CP violation,
which, if observed, could provide indirect indications of
unknown particles or interactions. To date, CP violation
has been established in transitions of strange and bottom
hadrons, with effects consistent with the SM predictions
[1,2]. Studies of CP violation in the interactions of charm
quarks offer a unique probe for non-SM physics. Charm
transitions are complementary to the processes involving K
and B mesons in that heavy up-type quarks (charge þ2=3)
are present in the initial state. Therefore, measurements of
CP violation in charm probe the presence of down-type
(charge −1=3) non-SM physics through charged-current
couplings [3]. Because charm transitions are well described
by the physics of the first two quark generations, CPviolating effects are expected not to exceed Oð10−2 Þ in the
SM [3]. Indeed, no CP violation has been experimentally
established yet in charm-quark dynamics [1].
Decay-time-dependent rate asymmetries of Cabibbosuppressed decays into CP eigenstates, such as D →
hþ h− , where D indicates a D0 or D̄0 meson, and h a K
or π meson, are among the most sensitive probes for CP
violation in this sector [4]. Such asymmetries,
ACP ðtÞ ¼
dΓðD0 → hþ h− Þ=dt − dΓðD̄0 → hþ h− Þ=dt
;
dΓðD0 → hþ h− Þ=dt þ dΓðD̄0 → hþ h− Þ=dt
ð1Þ
probe non-SM physics contributions in the oscillation and
penguin transition amplitudes. Oscillations indicate D0 –D̄0
transitions governed by the exchange of virtual heavy
particles occurring before the decay. Penguin decays are
second-order transitions mediated by an internal loop.
Either amplitude may be affected by the exchange of
non-SM particles, which could enhance the magnitude
of the observed CP violation with respect to the SM
expectation. The asymmetry ACP ðtÞ thus receives contributions from any difference between D0 and D̄0 decay
amplitudes (direct CP violation) and from any difference in
oscillation probabilities between charm and anticharm
mesons or interference between decays that follow or do
not follow an oscillation (indirect CP violation). Because
of the slow oscillation rate of charm mesons [1], Eq. (1) is
approximated to first order as [5]
t
þ −
þ −
ACP ðtÞ ≈ Adir
CP ðh h Þ − AΓ ðh h Þ;
τ
ð2Þ
where t is the proper decay time and τ is the CP-averaged
D-meson lifetime [6]. The first term arises from direct CP
violation and depends on the decay mode; the second term
is proportional to the asymmetry between the effective
lifetimes τ̂ of anticharm and charm mesons,
AΓ ¼
τ̂ðD̄0 → hþ h− Þ − τ̂ðD0 → hþ h− Þ
;
τ̂ðD̄0 → hþ h− Þ þ τ̂ðD0 → hþ h− Þ
and is mostly due to indirect CP violation [7]. Effective
lifetimes are defined as those resulting from a singleexponential fit of the time evolution of neutral meson
decays that may undergo oscillations. In the SM, AΓ is
universal for all final states with the same CP-parity [8],
such as K þ K − and π þ π − ; contributions from non-SM
processes may introduce channel-specific differences.
Measurements have been reported from electron-positron
collisions at the ϒð4SÞ resonance [9] and from high-energy
proton-proton collisions [10]. All results are consistent
with the hypothesis of CP symmetry with Oð10−3 Þ
uncertainties.
Any independent measurement of comparable precision
further constrains the phenomenological bounds and may
improve the knowledge of CP violation in the charm sector.
Decays D → hþ h− are well suited for a measurement of AΓ
at the Collider Detector at Fermilab (CDF). Fully reconstructed final states provide a precise determination of the
decay time, and large signal yields with moderate backgrounds allow for reduced systematic uncertainties.
In this paper, we report a measurement of CP-violating
asymmetries between the effective lifetimes of anticharm
and charm mesons reconstructed in D0 → K þ K − and
D0 → π þ π − decays. We use the full data set from
1.96 TeV proton-antiproton collisions collected by the
online event-selection system (trigger) on charged particles
displaced from the primary collision and corresponding
to 9.7 fb−1 of integrated luminosity. The analysis uses
D-meson candidates produced in the decay of an identified
Dþ or D− meson to determine whether the decaying state
was initially produced as a D0 or a D̄0 meson. Flavor
conservation in the strong-interaction processes Dþ →
−
D0 π þ
→ D̄0 π −s allows for the identification of the
s and D
initial flavor through the charge of the low-momentum π
meson (soft pion, π s ). Each decay-mode sample is divided
into subsamples according to production flavor and decay
time. In each subsample, a fit to the Dπ s mass distribution
is used to determine the relative proportions of signal and
background. These proportions are used to construct a
background-subtracted distribution of the D impact parameter, the minimum distance from the beam of the D
trajectory. This distribution is fit to identify D mesons
from b-hadron decays (secondary decays), whose observed
decay-time distribution is biased by the additional decay
length of the b hadron, and to determine the yields of charm
(N D0 ) and anticharm (N D̄0 ) mesons directly produced in the
pp̄ collision (primary decays). The yields are combined
into the asymmetry A ¼ ðN D0 − N D̄0 Þ=ðN D0 þ N D̄0 Þ,
which is fit according to Eq. (2). The slope yields AΓ .
111103-4
RAPID COMMUNICATIONS
MEASUREMENT OF INDIRECT CP-VIOLATING …
PHYSICAL REVIEW D 90, 111103(R) (2014)
The intercept determines the asymmetry at t ¼ 0, Að0Þ,
which receives contributions from direct CP violation and
possible instrumental asymmetries. We check that the latter
are constant in decay time using a low-background control
sample of 13 × 106 D → Dð→ K ∓ π Þπ s signal decays.
Sample selection, studies of background composition, and
fit modeling follow previous measurements [5,11].
The CDF II detector is a multipurpose magnetic spectrometer surrounded by calorimeters and muon detectors.
The detector components relevant for this analysis are
outlined as follows; a detailed description is in Ref. [5]. A
silicon microstrip vertex detector and a cylindrical drift
chamber immersed in a 1.4 T axial magnetic field allow for
the reconstruction of charged-particle trajectories (tracks)
in the pseudorapidity range jηj < 1. The vertex detector
contains seven concentric layers of single- and doublesided silicon sensors at radii between 1.5 and 22 cm, each
providing a position measurement with up to 15ð70Þ μm
resolution in the azimuthal (proton-beam) direction [12].
The drift chamber has 96 measurement layers, between 40
and 137 cm in radius, organized into alternating axial and
2° stereo superlayers [13]. The component of a chargedparticle momentum transverse to the beam (pT ) is determined with a resolution of σ pT =p2T ≈ 0.07% ðGeV=cÞ−1 ,
corresponding to a typical mass resolution of 8 MeV=c2 for
a two-body charm-meson decay.
The data are collected by a three-level trigger. At level 1,
custom hardware processors reconstruct tracks in the transverse plane of the drift chamber [14]. Two oppositely
charged particles are required, with reconstructed
transverse
P
momenta pT > 2 GeV=c, scalar sum pT > 5.5 GeV=c,
and azimuthal opening angle Δϕ < 90°. At level 2, driftchamber tracks are combined with silicon-detector hits
and their impact parameters (transverse distances of
closest approach to the beam line) are determined with
45 μm resolution (including the beam spread) [15] and
required to be between 0.12 and 1.0 mm. A more stringent
opening-angle requirement of 2° < Δϕ < 90° is also
applied. Each track pair is then used to form a D-meson
candidate, whose flight distance in the transverse plane
projected onto the transverse momentum (Lxy ) is required to
exceed 200 μm. At level 3, the selection is reapplied on
events fully reconstructed by an array of commercial
processors.
The offline reconstruction of signal candidates is solely
based on tracking information, without using particle
identification. Two tracks from oppositely charged particles
compatible with the trigger requirements are combined,
with pion or kaon assignment, in a kinematic fit to a
common decay vertex to form a D candidate. A charged
particle with pT > 400 MeV=c is associated with each D
candidate to form D candidates. We improve the
reconstruction with respect to Ref. [11] by using the
position of the beam as a constraint in the fit of the D
decay and retain only candidates with good fit quality.
Since the beam position is determined more accurately than
the trajectory of the soft pion, this provides a 25%
improvement in D mass resolution. Other offline requirements are based on a more accurate determination of the
quantities used in the trigger and are detailed in Ref. [11].
The D → K þ K − and D → π þ π − samples are separated by
requiring the selected candidates to have the relevant hþ h−
mass within 24 MeV=c2 of the known D mass, mD [6].
We reconstruct 6.1 × 105 D0 → K þ K − , 6.3 × 105
D̄0 → K þ K − , 2.9 × 105 D0 → π þ π − , and 3.0 × 105 D̄0 →
π þ π − signal decays (Fig. 1). The composition of the π þ π −
sample is dominated by the signal of D -tagged D decays
and a background of real D decays associated with random
pions or random combinations of three tracks (combinatorics). In the K þ K − sample, an additional background is
contributed by misreconstructed multibody charm-meson
decays, dominated by D0 → h− π þ π 0 and the D0 →
h− lþ νl contributions, where l is a muon or an electron.
Each decay-mode sample is divided into charm and
anticharm subsamples and into 30 bins of decay time
between 0.15τ and 20τ, chosen so that each contains
approximately the same number of candidates. The D
decay time is determined as t ¼ Lxy mD =pT , with approximately 0.2τ resolution, independent of decay time. The
observed decay-time distribution is biased by the trigger.
The effect of the bias is assumed to be independent of the
D-meson flavor and is accounted for when integrating
Eq. (2) over each decay-time bin.
Relative proportions between signal and background
yields in the signal region are determined in each decaytime bin, and for each flavor, through χ 2 fits of the Dπ s
mass distributions. The Dπ s mass is calculated using the
vector sum of the momenta of the three particles to
determine the D momentum and the known D and
charged π-meson masses [6]. The signal shapes are
determined from the sample of D → K ∓ π decays; the
parameters of the background shapes [5] are determined by
the fit. All mass shapes are determined independently for
each flavor and decay-time bin. The fit allows for asymmetries between combinatorial and misreconstructed background event yields, respectively, of the Dþ and D−
samples. The resulting shapes and background proportions
are used to derive signal-only distributions of the D-meson
impact parameter in each bin and for each flavor.
The impact parameter distributions of the sum of signal
and background components are formed by restricting the
2
analysis to candidates with MðDπ s Þ within 2.4 MeV=c of
the known D mass [6]. From these, we subtract the
impact parameter distribution of the background, derived
from the 2.015 < MðDπ Þ < 2.020 GeV=c2 region for the
π þ π − sample. The additional contamination from multibody decays in the K þ K − sample requires choosing a
suitable sideband that contains the same admixture of
combinatorial and misreconstructed backgrounds as that
expected in the signal region. We select as background the
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T. AALTONEN et al.
PHYSICAL REVIEW D 90, 111103(R) (2014)
×103
70
0
(b) D*−→ D (→ K +K −) π−s
(a) D*+→ D0(→ K +K −) π+s
60
Data (9.7 fb-1)
Candidates per 0.09 MeV/c 2
50
Fit
40
D → multibody
30
Combinatorics
20
10
30
0
(d) D*−→ D (→ π+π−) π−s
(c) D*+→ D0(→ π+π−) π+s
25
20
15
10
5
0
2.005
2.01
2.015
2.005
2.01
2.015
2.02
Dπ±s mass [GeV/c 2 ]
0
þ −
0
þ −
FIG. 1 (color online). Distributions of Dπ s mass with fit results overlaid for (a) the D → K K sample, (b) the D̄ → K K sample,
0
þ
−
0
þ
−
(c) the D → π π sample, and (d) the D̄ → π π sample.
candidates with mD − 64 MeV=c2 < MðK þ K − Þ < mD −
2
40 MeV=c2 and with MðDπ s Þ within 2.4 MeV=c of the
known D mass. Checks on data show that the final results
are robust against variations of these choices. We perform a
χ 2 fit of the background-subtracted impact-parameter distribution of D candidates in each subsample of decay time
and flavor, using double-Gaussian models for both the
primary and secondary components. Since we determine
impact parameters using information associated with the D
decay only, the shapes of the impact-parameter distributions of D0 and D̄0 mesons are consistent. The parameters
of the primary component are fixed in all fits. They are
derived from fits of candidates in the first decay-time bin
(t=τ < 1.18), where any bias from the Oð%Þ secondary
contamination is negligible, as supported by repeating the
fit using an alternative model derived from the second bin
and observing no significant difference in the results. The
parameters of the secondary component are determined by
the fit independently for each decay-time bin. Example
impact-parameter fits are shown in Fig. 2. All mass and
impact-parameter fits show good agreement with data.
Extreme variations of model parameters yield large changes
in the fit χ 2 but negligible changes in the results.
Final χ 2 fits of the asymmetries between the resulting
yields of primary charm and anticharm decays as functions
of decay time are used to determine the values of AΓ in the
two samples. The fits are shown in Fig. 3 and yield
AΓ ðK þ K − Þ ¼ ð−0.19 0.15ðstatÞÞ% and AΓ ðπ þ π − Þ ¼
ð−0.01 0.18ðstatÞÞ%. The value of χ 2 divided by the
number of degrees of freedom is 28=28 in both fits. In both
samples we observe Að0Þ ≈ −2%, due to the known
detector-induced asymmetry in the soft-pion reconstruction
efficiency [5]. The independence of instrumental asymmetries from decay time is checked by performing the analysis
on D → K ∓ π decays, where no indirect CP violation
occurs and instrumental asymmetries are larger due to the
additional effect from the difference in interaction probability with matter of opposite-charge kaons; an asymmetry
slope compatible with zero is found, ð−0.5 0.3Þ × 10−3 .
The width of the impact-parameter distribution of primary
D mesons increases as a function of decay time, as
predicted in simulation. This has no significant effect on
AΓ , as verified by repeating the measurement with a
floating width that increases linearly with decay time.
The dominant systematic uncertainty in the measurement
of AΓ ðπ þ π − Þarises from the contribution of 0.028% from
the choice of the impact-parameter shape (single- or
double-Gaussian function) of the secondary component
whereas for AΓ ðK þ K − Þ this effect contributes a smaller
uncertainty of 0.013%. The choice of the background
sideband has a dominant effect in the K þ K − analysis
(0.038%) and a minor impact (0.010%) on the π þ π −
result. Other minor effects are associated with the uncertainty on the vertex-detector length scale (0.001% to
0.002%); the neglected 0.93% contamination of misreconstructed K − π þ decays in the π þ π − sample (< 0.001%);
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MEASUREMENT OF INDIRECT CP-VIOLATING …
+
D * → D (→ π+π−) π+s
350
(a) 2.08 < t /τ < 2.16
0.04
Data (9.7 fb-1)
Fit
0
400
Secondary decays
300
250
CP violation allowed
-0.02
-0.04
150
-0.06
100
No CP violation
0
200
0.04
50
(b)
0.02
200
180
A (π+π −)
Candidates per 2.5 μm
Data (9.7 fb-1)
(a)
0.02
A (K +K −)
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PHYSICAL REVIEW D 90, 111103(R) (2014)
(b) 6.16 < t /τ < 20
160
0
-0.02
-0.04
140
120
-0.06
100
0
80
2
4
t /τ
6
20
60
FIG. 3 (color online). Effective lifetime asymmetries as functions of decay time for the (a) D → K þ K − and (b) D → π þ π −
samples. In each bin, the position of the data point corresponds to
the average decay time in that bin. Results of fits not allowing for
(dotted line) and allowing for (solid line) CP violation are
overlaid.
40
20
0
-0.04
-0.02
0
0.02
0.04
D 0 impact parameter [cm]
FIG. 2 (color online). Distributions of the D-meson impact
parameter with fit results overlaid for background-subtracted
D → π þ π − decays restricted to (a) the decay-time bin 2.08 <
t=τ < 2.16 and (b) the decay-time bin 6.16 < t=τ < 20. Similar
distributions are observed for D → K þ K − decays.
the neglected bin-by-bin migration due to the decay-time
resolution (< 0.001%); and any possible fit biases
(< 0.001%), probed by repeating the analysis on the
π þ π − sample with random flavor assignment.
In summary, we measure the difference in the effective
lifetime between anticharm and charm mesons reconstructed in D0 → K þ K − and D0 → π þ π − decays using
the full CDF data set. The final results,
AΓ ðK þ K − Þ ¼ ð−0.19 0.15ðstatÞ 0.04ðsystÞÞ%;
AΓ ðπ þ π − Þ ¼ ð−0.01 0.18ðstatÞ 0.03ðsystÞÞ%;
are consistent with the hypothesis of CP symmetry. Their
combination yields AΓ ¼ ð−0.12 0.12Þ%, assuming that
uncertainties are uncorrelated. The results are consistent
with the current best determinations [9,10] and improve the
global constraints on indirect CP violation in charm-meson
dynamics.
We thank the Fermilab staff and the technical staffs of the
participating institutions for their vital contributions. This
work was supported by the U.S. Department of Energy and
National Science Foundation; the Italian Istituto Nazionale
di Fisica Nucleare; the Ministry of Education, Culture,
Sports, Science and Technology of Japan; the Natural
Sciences and Engineering Research Council of Canada;
the National Science Council of the Republic of China; the
Swiss National Science Foundation; the A. P. Sloan
Foundation; the Bundesministerium für Bildung und
Forschung, Germany; the Korean World Class University
Program, the National Research Foundation of Korea; the
Science and Technology Facilities Council and the Royal
Society, United Kingdom; the Russian Foundation for
Basic Research; the Ministerio de Ciencia e Innovación,
and Programa Consolider-Ingenio 2010, Spain; the Slovak
R&D Agency; the Academy of Finland; the Australian
Research Council (ARC); and the EU community Marie
Curie Fellowship Contract No. 302103.
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PHYSICAL REVIEW D 90, 111103(R) (2014)
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